Warren has already made clear that she's not afraid of going after Trump, and she said Thursday she's prepared to serve as vice president if she were chosen as Clinton's running mate.

"Pocahontas is at it again!" Trump tweeted in apparent response Friday. "Goofy Elizabeth Warren, one of the least productive U.S. Senators, has a nasty mouth. Hope she is V.P. choice."

Warren endorsed Clinton for president on MSNBC on Thursday.

While she has considered serving as Clinton's running mate, she reportedly has not discussed the idea with Clinton, sources familiar with Warren's thinking told Reuters.

But Warren has been arguably the most effective critic of Trump on the Democratic side. She has blasted the brash billionaire on Twitter several times, and she gave a fiery speech on Thursday in which she called Trump a "loud, nasty, thin-skinned fraud" and a "racist bully."

She lobbed a fresh round of insults at Trump during her MSNBC appearance.

"Donald Trump is a genuine threat to this country," she told Rachel Maddow. "He is a threat to who we are as a people."

On Friday morning, she responded to Trump's tweet by echoing Clinton's sentiments from the day before.